Comparative study of the protective effect against Salmonella colonisation in newly hatched SPF chickens using live, attenuated Salmonella vaccine strains, wild-type Salmonella strains or a competitive exclusion product.

There is a need to prevent intestinal colonisation by Salmonella enteritidis and S. typhimurium in newly hatched chicks. Treatment with an undefined bacterial flora is not acceptable to regulatory agencies in some countries because of the potential risk of transmitting pathogens. A defined culture with a potency and stability equivalent to those of an undefined culture has not yet been developed. Since attenuated Salmonella vaccine strains could possess the colonisation characteristics but not the virulence of Salmonella wild-type strains, they could inhibit colonisation of the challenge organism. S. typhimurium live vaccines registered in Germany (Zoosaloral H, Salmonella vac T), S. enteritidis aroA and S. typhimurium aroA strains, S. enteritidis, S. typhimurium and S. infantis wild-type strains or a competitive exclusion product (Broilact) were used as pretreatment cultures and evaluated for their inhibitory effects against S. enteritidis and S. typhimurium colonisation in newly hatched SPF chickens. Day-old chicks were administered a pretreatment culture and infected orally with variants of S. enteritidis or S. typhimurium wild type-strains resistant to nalidixic acid or rifampicin 1 day after pretreatment. On days 2 and 6 after infection, viable numbers of the challenge strain in liver and caeca were determined. The results for birds pretreated with Broilact showed a distinct protective effect against both S. enteritidis and S. typhimurium at a challenge dose of 10(4) cfu/bird. After pretreatment of chicks with S. enteritidis and S. typhimurium wild-type strains, the greatest degree of inhibition of caecal colonisation was produced using isogenic strains. Colonisation after infection with non-isogenic strains could not be prevented but only reduced for a brief period. These effects were also observed after administration of aroA strains of S. enteritidis and S. typhimurium but the protective effect was considerably lower than after pretreatment with wild-type Salmonella strains. Inoculation with attenuated S. typhimurium vaccines resulted in a weak but significantly reduced colonisation by S. typhimurium. Colonisation by S. enteritidis could not be diminished by either of the S. typhimurium vaccine strains. The results indicate in principle the potency of Salmonella vaccine strains to inhibit Salmonella wild-type colonisation in newly hatched chicks. Potential vaccine candidates should be tested for their capacity to prevent intestinal colonisation in newly hatched chicks.